Air-cooled packaged roof top air conditioning units are perhaps the most ubiquitous type of Heating, Ventilating and Air Conditioning (HVAC) units utilized today in commercial and industrial facilities in the United States of America. This is due to their relative ease of installation and low equipment and installation cost. The down side of these units is their inherent inefficiency in terms of electrical energy consumption. These units are also higher in average maintenance costs due to compressor and condenser fan failures compared to central refrigeration and chilled water systems. This is due in part to the manufactures desire to remain competitive in markets that require low initial equipment cost.
Air-cooled packaged roof top air conditioning heat pump units are more prevalent in the southern climates due to milder winters, whereas air-cooled packaged rooftop air conditioning units with natural gas fired heating sections are more prevalent in more northerly climates due to the relatively higher heating demands these climates present.
The typical air cooled rooftop air conditioning unit contains the following major components; a supply (evaporator) fan for delivering the cooled or heated and ventilated air to the air conditioned space, an air filter section, one or multiple condenser fans for conveying ambient outdoor air across the air cooled condenser coil, one or more refrigerant compressors to drive the refrigerant cooling or heat pump cycle, a condenser coil and evaporator coil, and expansion device to regulate the flow and pressure drop of liquid refrigerant as it travels from the condenser and/or condenser liquid receiver into the evaporator coil, the main equipment housing, and the electrical and controls components. Some units also contain automated outdoor air, return air, and exhaust air dampers while others do not. Of these typical components the most common source of major maintenance cost is system failure due to failure of the condenser fan or a compressor failure. Condenser fan failures can often lead to elevated refrigeration condensing pressures (or head pressures) which causes stress on the compressor that can then lead to a costly compressor failure and equipment downtime.
For facilities with multiple RTUs the frequency of compressor or other major failures increases with the number of units per facility. In other words a facility with 20 RTUs containing 2 compressors each has a total of 40 compressors. If these units are all at their mean expected life, one could expect a compressor to fail on a typical unit once every 8 to 10 years. However since there are 40 compressors one could expect 4.4 compressors failing on average every year. In today's market a typically sized compressor replacement cost could be expected to be on the order of $5,000 to $7,000 each for a total average compressor replacement cost of approximately of $22,000 to $30,800 per year.
In addition to high maintenance costs a typical rooftop unit will have a cooling efficiency on the order of 1.0 to 1.2 kW/ton of cooling. The cooling efficiency of an air-cooled rooftop air conditioning unit is inherently limited due in large part to the dry air-cooled condenser coil. An air cooled condenser coil rejects heat to the air and therefore must be operated at a higher temperature than the surrounding ambient outdoor air such that heat transfer can take place. For facility cooling systems the peak cooling system loads are often coincident with the warmest outdoor air temperatures and the times with the highest solar radiation heat load on the facilities roofs, walls, and windows. At these times the RTUs are likely to experience high ambient outdoor air temperatures when they are also experiencing their peak cooling load demands. The primary disadvantage of an air-cooled RTU is that it is inherently limited in its efficiency by the amount of “lift” that the compressor has to perform in terms of pressure increase of the refrigerant vapor from the relatively low pressure of the evaporator coil to the relatively high pressure of the condenser coil. When it is warm outside the refrigerant compressor needs to compress the refrigerant vapor to a higher pressure to drive the heat rejection process to allow the refrigerant to condense so that it can be fed to the expansion device for subsequent liquid to vapor phase change in the evaporator (cooling) coil. By design RTUs are typically located on the roofs of facilities where they are exposed to high ambient temperatures as a result of solar heating of the rooftop surfaces which cause further efficiency losses due to high ambient air temperatures. These high ambient temperatures not only increase energy use but also cause the compressor to have to work harder per unit of cooling energy thus causing further stress and potentially reduced compressor equipment life.
In contrast to the typical RTU based cooling systems, central chilled water systems or central refrigeration systems are typically water cooled via a cooling tower system or evaporative condenser based systems. New water cooled refrigerating or chilling equipment typically have efficiencies on the order of 0.6 to 0.7 kW/ton of cooling, which is on the order of 40% less energy consumption than their packaged air cooled rooftop counterparts. The primary advantage of a water cooled systems is in the fact that by being water cooled the temperature at which they are able to reject heat is always going to be lower than or at worst case, the same as that of an equivalent air-cooled refrigeration cycle. This is due to the fact that water cooled system refrigerant condensing pressures are a function of the ambient wet-bulb temperatures which have to do with the relative dryness or wetness of the outdoor air surrounding the heat rejection equipment and are not so much a function of the air temperature its self. Air wet-bulb temperatures are always less than or in the extreme case equal to the dry-bulb temperature of that same air. The only time the wet-bulb and the dry-bulb air temperatures are equal is when the air is 100% saturated with water, or is at a state of 100% relative humidity, which is an upper limit case. Depending on local climate conditions a typical water cooled system can be expected to operate at approximately 40% less energy consumption per unit of cooling than an otherwise equivalent air cooled system.
Traditional central cooling system design for the HVAC industry involves creating chilled water loops and pumping chilled water out to chilled water air cooling coils located in the facilities air handling units. These can be units located inside the building or located on the rooftop, or any combination thereof. The down side to this approach is the water must be chemically treated to scavenge oxygen, prevent biological growth, and prevent corrosion and in some systems prevent freezing. The other downside of central pumping systems is the added temperature approach of dual heat transfer surfaces between the actual air stream to the chilled water stream and then a second heat exchange from the chilled water stream to the refrigerant in the water chilling machine. This results in increased energy as a result of added temperature differential between the working fluid, which is the refrigerant and the medium to be cooled, which is the buildings HVAC system air stream in the air handling unit(s). Chilled water systems are also require much larger pipes, and significantly increased pumping power as compared to a circulating cooling medium such as a refrigerant which is undergoing a phase change and is therefore capable of moving significantly more Btu's per lb. of circulating refrigerant than that of a chilled water system, thereby reducing pumping costs and piping material costs.
Another point to consider is that central chilled water of refrigeration system equipment is typically much larger cooling capacity equipment than would be found in a typical rooftop unit. Central refrigerating or cooling equipment is typically on in the size range of 200 to 1,000 tons of cooling capacity vs. a typical RTU compressor is typically in the cooling capacity size range 5 to 30 tons of cooling capacity. The larger capacity equipment found in central systems are typically a much larger investment and as such are less of a commodity grade type of equipment when compared to a compressor package in a typical RTU. In central systems the facility owner or design engineer is typically responsible for selecting the compressor systems that go into the cooling system design. In RTUs the equipment manufacturer, who is more driven by market forces focused on low first equipment cost, is the party whom selects and installs the compressor system which is delivered as part of their RTU package. Due to these market conditions centralized systems tend to be built with the end consumer in mind vs. the mass producer of RTUs and as such life expectancy and efficiency are a stronger driving market force that in the RTU market. In the RTU market the smaller compressors are at the point of being more or less a consumable items often with little or no serviceable components, that upon failure the entire unit is simply replaced, versus in the central chilling or refrigeration market, equipment is designed and built to be highly efficient and equipment is designed to last on the order of 15 to 25 years with serviceable components. Centralized refrigeration or chilled water equipment is not designed to be low cost disposable types of equipment to the extent that is done in the RTU market.
Added efficiency can be gained for facilities with diverse heating and cooling zones, whereby heat from zones which are requiring cooling could be operated off of the evaporator side of the heat pump system while at the same time for zones that require heating energy can operate as the condenser when in heat pump mode. In this way heat is removed from the warm zones which are requiring cooling and this heat is then transferred to other zones which are requiring additional heat or reheat.
All of these aforementioned items translate into increased reliability, efficiency, and lower maintenance costs for centralized water cooled systems compared to typical air-cooled RTU based cooling systems.